The present invention relates generally to the formation of antenna patterns through spatial processing and more particularly to a method for minimizing code and carrier multipath signal errors when tracking the GPS signals.
Phased array antennas are used in a variety of aerospace applications. A phased array antenna has a number of antenna elements that are aligned in phase to provide transmit or receive gain. By adjusting the amplitude and phase of the input signals from the different antenna elements using complex weights, interference sources can be isolated and rejected from the composite signal, and the desired signal can be reinforced. Phased array antennas have been used to eliminate interference sources for GPS (Global Positioning Satellite) receivers or to increase the received signal power through beam steering to the GPS satellites. It is the purpose of the present invention to use the phased array to minimize the effect of multipath signal sources in a GPS receiver""s tracking loops.
Multipath errors are caused by the receiver tracking a composite of the direct GPS signals and GPS signals reflected from nearby objects. The resulting phase error is a function of the phase offset between the direct and multipath signals and the relative signal strength. For a fixed installation, these errors appear as biases, changing only as the line-of-sight to the satellite changes due to the satellite motion. In a mobile application, the multipath errors will also change due to the motion of the antenna.
GPS receivers commonly use broad antenna gain patterns so that all of the satellites above the horizon can be tracked. Unfortunately, this increases the susceptibility of GPS receivers to multipath reflections of the GPS signals from nearby objects. Multipath signals from below the receiver antenna can be removed using some types of antenna design, such as a choke ring, but signals arriving form elevations above the antenna cannot be rejected without also eliminating the satellite signals that are needed to obtain high accuracy GPS solutions. A simple depiction of multipath signals received at a GPS antenna is shown in FIG. 1.
Since the GPS receiver observes a composite signal, including both the direct path and the indirect (multipath) path, the code and carrier tracking loops track the composite signal, resulting in pseudo-range (code) errors and carrier-phase errors which are proportional to the additive multipath path length and the relative received power of the multipath signals to the direct signal from the satellite.
Numerous signal processing techniques have been proposed in the prior art for reducing the effect of multipath errors on the tracking loops. Exemplary of this prior art are U.S. Pat. No. 5,347,536 to Meehan and U.S. Pat. No. 5,414,729 to Fenton. These references are directed to methods employing temporal filtering of the multipath errors, multipath correction from site calibration or correlation shape correction, and multiple digital correlators. These techniques can be used to reduce the effect of multipath on the GPS code and carrier tracking loops but do not remove the source of the error. It would be advantageous to remove the multipath error source before the GPS tracking loops are employed.
Briefly, the present invention is directed to a GPS phased array and a digital GPS receiver which includes electronics to detect the presence of multipath signals, determine the direction from which they arrive, and adaptively generate an antenna pattern to provide gain in the direction of the desired satellite signal and to apply null signals in the direction of the detected multipath signals. This adaptively-generated antenna pattern is applied to the signals from the multiple antenna array elements to provide a composite signal to each of the GPS receiver processing channels optimized for the particular satellite being tracked by that channel.
The advantage of this technique is that the multipath signals are excised from the inputs to the GPS receiver processing channels, and the direct signal is reinforced, thereby enabling the GPS receiver tracking loops to make highly accurate observations of the code and carrier phase using conventional signal processing techniques.